1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus including the same, which are suitable for a broadcast television camera, a video camera, a digital still camera, a silver halide camera, and the like.
2. Description of the Related Art
In recent years, a zoom lens system having a high zoom ratio and high optical performance has been demanded for an image pickup apparatus such as a television camera. As a method of realizing a high zoom ratio, there is known a built-in extender system. In the built-in extender system, a partial system constituting a relay portion that constitutes the zoom lens system and is fixed for zooming is removed from the optical path, and an extender is inserted in place of the partial system, to thereby shift a focal length range of the entire system.
For instance, there is known a four-unit zoom lens system including a first lens unit for focusing, a second lens unit for varying magnification, a third lens unit for correcting image plane variation, and a fourth lens unit (relay portion) for imaging that includes a lens unit exchangeable with an extender (see U.S. Pat. No. 5,097,360 or Japanese Patent Application Laid-Open No. 2009-098419). In this zoom lens system disclosed in U.S. Pat. No. 5,097,360 or Japanese Patent Application Laid-Open No. 2009-098419, the fourth lens unit is constituted of a fourth-first unit having negative optical power, a fourth-second unit that is insertable into and removable from the optical path, and a fourth-third unit having positive optical power. In this case, the fourth-second unit is exchanged with the extender so as to shift a focal length range of the entire system.
The zoom lens system that is used for an image pickup apparatus such as a television camera or a video camera is required to have a high zoom ratio and high optical performance, and further, the entire system is required to be small and light. In general, in the built-in extender system, it is necessary to secure a space having a predetermined length in the optical axis direction for inserting and removing the extender in the relay portion. For this reason, the entire length of the relay portion is apt to increase. In order to suppress the increase in the entire length of the relay portion, it is effective to reduce a length of the extender in the optical axis direction.
When the extender, which causes the focal length range of the entire system to shift to a long focal length side, is inserted in the optical path, an exit ray height from the extender is required to be smaller than an incident ray height to the extender. Therefore, in general, the extender is constituted of a front unit having positive optical power and a rear unit having negative optical power. In order to decrease the entire length of the extender, it is necessary to increase positive optical power of the front unit and to increase negative optical power of the rear unit. In general, if the front unit and the rear unit are constituted to have such large optical power, not only an absolute amount of aberration but also high order aberration increases. As a result, it becomes difficult to correct for aberration.
FIGS. 14A and 14B are explanatory diagrams of the four-unit zoom lens system in the case where a ray enters the fourth-first unit having negative optical power constituting a relay portion in a converging manner, and exits from the same in a diverging manner. FIG. 14A is a schematic diagram in which the fourth-second unit is a standard lens unit and the focal length range of the entire system is in a standard state. FIG. 14B is a schematic diagram in which a fourth-fourth unit is an extender and the focal length range of the entire system is shifted to the long focal length side as an extender switched state. When the ray exits from the fourth-first unit in a diverging manner, the incident ray height to the fourth-fourth unit becomes large. Therefore, as illustrated in FIG. 14B, it is necessary to increase a degree of decreasing the exit ray height with respect to the incident ray height to the fourth-fourth unit. Therefore, it is inevitable to increase the optical powers of the front unit and the rear unit constituting the fourth-fourth unit (extender). On the contrary, if the optical power of each unit is decreased, it is difficult to reduce the entire length of the fourth-fourth unit.
FIGS. 15A and 15B are explanatory diagrams of the four-unit zoom lens system in the case where a ray enters the fourth-first unit having negative optical power constituting the relay portion in a converging manner, and exits from the same in a converging manner. FIG. 15A is a schematic diagram in which the fourth-second unit is the standard lens unit and the focal length range of the entire system is in the standard state. FIG. 15B is a schematic diagram in which the fourth-fourth unit is an extender and the focal length range of the entire system is shifted to the long focal length side as the extender switched state.
If the ray exits from the fourth-first unit in a converging manner, the incident ray height to the fourth-fourth unit becomes small. Therefore, as illustrated in FIG. 15B, in the extender switched state, a degree of decreasing the exit ray height with respect to the incident ray height to the fourth-fourth unit is small. Therefore, it is advantageous for decreasing the entire length of the fourth-fourth unit. However, as illustrated in FIG. 15A, in the standard state, a degree of increasing the exit ray height with respect to the incident ray height to the fourth-second unit is large. Therefore, the optical power of the fourth-second unit becomes large, which makes it difficult correct for aberration in the standard state of the focal length range of the entire system. As described above, in order to secure high optical performance while reducing the entire length of the fourth-fourth unit (extender), it is necessary to appropriately set lens structures of the relay portion and the extender.